Hormones, brain, behavior, and the environment interact in a complex manner to adapt organisms to their specific environment. For example, studies with reptiles have proven these animals to be model systems for examining the neuroendocrine control of reproductive behavior in a vertebrate. In male lizards environmental changes (eg., increasing temperature) in the spring stimulate pituitary gonadotropin secretion. These hormones are transported by the bloodstream to the testes where they stimulate sperm prduction and androgen (testosterone) secretion. The rising concentration of testosterone in the blood feeds back onto specific areas of the brain to modulate further pituitary gonadotropin secretion. These hormones are transported by the bloodstream to the testes where they stimulate sperm production and androgen (testosterone) secretion. The rising concentration of testosterone in the blood feeds back onto specific areas of the brain to modulate further pituitary gonadotropin secretion and to activate male sexual behavior. The male's behavior then becomes an important aspect of the environment that, along with photothermal stimuli, stimulate goandotropin secretion in the conspecific female. As a result of male courtship behavior, ovarian development and hormone production are rapidly induced. During the breeding season, females exhibit cycles of sexual receptivity which influence the behavior, and indirectly the physiology, of conspecific males. In this manner, the successive stages of the reproductive cycle are controlled.